£300-£400M is quite plausible.
Worthy Down cost £250M and that contract was awarded over 5 years ago:
Worthy Down Barracks | Garrisons | Winchester-Garrison
Lyneham cost £230M and that was just the REME piece:
https://www.gov.uk/government/news/d...army-engineers
£42M for a single hangar for A400Ms:
https://www.gov.uk/government/news/a...f-brize-norton
£70M to build 322 families quarters:
https://www.gov.uk/government/news/w...es-at-tidworth
£25M for demolition and then £142M for construction of the support building for F35 at Marham:
https://www.gov.uk/government/news/m...-edge-aircraft
£20M to resurface just one runway at Valley:
https://www.gov.uk/government/news/r...-at-raf-valley
16x SLAM Blocks costs £77M at Brize (that would be roughly the amount needed at Cranwell):
https://www.gov.uk/government/upload...ic__Final_.pdf
£18M to renovate and build a new annex for 51 Sqn and RJ:
https://www.gov.uk/government/news/h...raf-waddington
Welcome to the world of defence infrastructure and so-called 'value for money'.
LJ